#app4 > .circle {
    width: 30vw;
    height: 30vw;
    border: 1px solid #ff0000;
    border-radius: 50%;
}
#app4 > .circle.active {
    animation: change 1s linear infinite alternate;
}
@keyframes change {
    0% { background-color: red; }
    100% { background-color: purple; }
}